Detection system

ABSTRACT

A detection system ( 10 ) utilising projected light ( 18, 20 ) viewed through an appropriately placed camera ( 22 ) detects variations in surface height ( 26 ). The system ( 10 ) is arranged to project light onto the surface ( 16 ) of interest. A camera with optical axis ( 24 ) oriented parallel to and offset from the direction of light projection is used to image the intersection ( 50, 66 ) of the projected light ( 18, 20 ) with the surface ( 16 ). The system ( 10 ) observes deflections ( 64 ) of this imaged intersection ( 66 ) and associates them with the movement of an object ( 26, 124 ) through the projected light. The object can range from a surface irregularity ( 124 ) to a person ( 26 ) walking along a corridor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The problem of building security is an important issue currently beingaddressed by developing technology. Automatic counting mechanisms fordetermining and/or controlling the number or identity of people passingthrough a particular entrance or exit have been around for some time.They vary from the simple automatic turnstile to swipe card access andradio-frequency tagging systems. A principal disadvantage of all theseprevious techniques is the low access speed. An automatic turnstile isparticularly obstructive in requiring considerable effort to be made bymoving personnel. None of the systems can be operated with a defaultunlocked door making a locking/unlocking mechanism unavoidable. In swipecard systems such door mechanisms are prone to failure. Radio-frequencytagging doesn't detect untagged intruders and so cannot be used tomaintain a default unlocked system.

There is a perceived need for a detection system capable of monitoringpersonnel movement which provides for a faster throughput of traffic.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a detection system capableof providing non-intrusive monitoring of personnel movement throughdefault unlocked entry and exit points.

The present invention provides a detection system including a lightsource arranged to project light onto an obstructing surface and imagingmeans arranged to selectively image the intersection of the projectedlight with the obstructing surface characterised in that the imagingmeans comprises at least one camera sensitive to light emitted from saidlight source and the imaging means and light source are mutuallyarranged such that the orientation of imaging means optical axis anddirection of light projection from said source are parallel or intersectat a position on the opposite side of the obstructing surface to theimaging means and source of projected light.

The invention provides the advantage of non-intrusive detection. Itexploits the effects of perspective on objects when viewed from aparticular direction: an object interrupting such projected light can bereadily detected as an outward movement of an imaged intersection in thefield of view. It thus allows for the detection of object movementthrough the system without obstruction of the traffic flow.

The system is preferably arranged to monitor the presence or absence ofbodies on a bounding surface intersecting the projected light. In thisregard, the bounding surface forms the obstructing surface in theabsence of a body; and the surface of the body forms the obstructingsurface in the presence of a body. The imaged intersection then hasdifferent positions according to whether it arises from the boundingsurface or the surface of the body and these positions are relativelydisplaced in accordance with the body surface's remoteness from thebounding surface. This provides the advantage of simplicity inapplication of the invention to numerous situations. The intersection ofthe projected light with a surface can be imaged and that imagemonitored continuously. Movement of the image can be ascribed to anobject on the surface passing through the projected light. The necessarymovement may be provided by the object itself, motion of a level surfacesupporting the object or by movement of the light sheets and imagingmeans across a stationary surface and objects. Moreover, the objectsneed not be physically separable from the surface the invention can beused to map the structure of the surface itself.

The projected light is preferably in the form of a substantially planarsheet of light and the imaging means is arranged such that its opticalaxis is substantially parallel to and offset from the plane of the lightsheet. This improves the detection capability of the system. Theintersection with a level surface will thus be a bright line which willform a well defined image. Any object crossing this bright line anywherealong its length will cause a deflection in the image. Furthermore thenarrowness of the line improves the capability of the system in itsprovision of accurate information regarding the height of aninterrupting object.

The projected light may be in the form of two substantially parallelplanar sheets of light disposed about the imaging means and the imagingmeans arranged such that its optical axis is substantially parallel tothe planes of the light sheets. This provides the capability fordetermining direction of travel through the system by means of observingthe sequence of light sheet disturbance.

The imaging means may be arranged to form an image of an intersection ofthe obstructing surface and a light sheet wherein the intersection isdetectable as a line in the image. The system may be arranged to respondto a deflection of such an image line. This provides an improvement toutility. The system is capable of responding to the data received andthe need for manual interpretation is reduced.

The system may be arranged to monitor the profile of bodies on abounding surface intersecting a projected light sheet. In the presenceof a body, the surface of the body forms the obstructing surface and theimage line defines a deflection pattern characteristic of the profile ofthe body. This increases the information available to the detectionsystem and renders it capable of more sophisticated responses.

The deflection pattern may comprise perpendicular displacements of imageline components from their original positions in the line. Thedisplacements (d_(shift1)) measured at the image plane of the imagingmeans are described by the equation$d_{{shift}\bot} = {{fd}_{\bot}\frac{h_{body}}{h_{c}( {h_{c} - h_{body}} )}}$

where d₁ is the perpendicular distance from the imaging means to thelight sheet responsible for the line in the image, f is the focal lengthof the imaging means₁ h_(c) is the distance between the imaging meansand the bounding surface and h_(body) is a parameter describing the bodyheight at each point that it intersects the light sheet. This provides astraightforward means of deriving the profile of an interrupting objectfrom the deflection pattern observed as the object passes through alight sheet.

The projected light may be directed transversely of a longitudinallyextending transit zone, and projected from a bounding region in whichthe imaging means is located. This provides for complete coverage acrossthe transit zone and the system is therefore capable of detecting anyobject moving along it. In one embodiment the transit zone may be acorridor, the obstructing surface is the corridor floor or the surfaceof a body and the bounding region is the corridor ceiling. This providesa detection system which is capable of monitoring personnel movementinto and out of a designated room or area of a building.

The imaging means may be a single camera or a one-dimensional array ofat least two cameras, the array alignment being substantiallyperpendicular to the component cameras' optical axes and substantiallyparallel to the plane of the projected light. A single camera providesthe advantage of cost reduction but an array will allow imaging alongthe length of the light sheets without demanding an extensive singlecamera field of view. Furthermore the array also provides an improvementin accurate counting. A single camera could find its view of one bodyobscured by a second, nearer body. This is particularly likely to occurif the second body is taller than the first. The effects of suchobscuration are largely overcome by the use of a number of cameras asany one body will be imaged in at least one camera.

The light source may be at least one strip source extendinglongitudinally along one side of each light sheet. Alternatively, it maycomprise an array of point sources for each sheet such that the array islocated along one side of each light sheet and arranged to project lightto the opposite side of the respective light sheet. Each point sourcemay be associated with a cylindrical lens and thereby arranged toproject light fanned within the plane of the respective light sheet. Astrip source provides the advantage of security. Objects passing throughthe light sheet will disturb the light regardless of their positionwithin the sheet. It also enables a profile of the object to be derivedfrom the deflection pattern without interpolation. Fanning an array ofcollimated point sources will also enable complete transverse coverage,and this implementation is more cost effective. An unfanned collimatedarray will require more sources in order to achieve effective transversecoverage but it does reduce the problems of potential obscuration. Inthe fanned case, light may be blocked from illuminating a body by asecond, closer body. Furthermore, using an unfanned collimated arraysimplifies both the computation associated with the invention and theimplementation of its optics. With such a collimated array the projectedlight is imaged as a series of two-dimensional dots as opposed to aline. The locus of deflection of each dot is known and so only a limitednumber of pixels need to be searched in locating the deflection pattern.Moreover, a dot exhibits a two-dimensional intensity profile whichincreases the reliability of its detection.

The imaging means may be focused on a plane located within a heightrange 1.6 m to 2.0 m above the floor. This range is the average humanheight and so this feature enhances the focus of the deflection patternin any implementation for which the system detects human traffic.

The imaging means is preferably connected to a data processing systemresponsive to the form and/or change in the image formed by the imagingmeans. This provides the system with a powerful capability to interpretand react to data received from the imaged intersections.

The data processing system may be arranged to respond with a count eachtime a deflection of the image line occurs. In embodiments employing twosheets of light, the data processing system is arranged to associate adeflection of one image line from a first position in the image toanother position with a subsequent deflection of the other image linefor the purpose of determining direction of travel of the body causingsaid deflections and further arranged to respond with a count on theoccurrence of such a pair of deflections. In this way the system iscapable of keeping an account of the number of personnel occupying adesignated area guarded by the detection system of the invention. Thisis advantageous to the use of the system in secure or restricted accessbuildings.

A first deflection of one image line may be associated with a deflectionof the other image line which occurs most immediately after the firstdeflection. Or, a deflection of one image line may be associated with adeflection of the other image line which occurs nearest to apredetermined time after the first deflection. These embodiments providemethods of association which are not particularly demanding of computingpower in their application. Alternatively, the data processing systemmay be arranged to apply pattern matching techniques to match adeflection pattern at one image line with a deflection pattern at theother image line and thereby to associate said two deflections. Thisprovides an increase in accuracy while monitoring two-directional flowof traffic. A body is not counted until its profile is registered firstat one image line and then at the other.

The data processing system includes an image processor arranged torecord, process and digitise the deflection pattern, a counting unitarranged to count the number of such deflection patterns occurring ineach light sheet, an interpreter arranged to associate disturbancesarising from the same body passing through both light sheets and amemory arranged to provide data to the interpreter. This provides astraightforward example of a data processing system suitable forimplementation with the projected light detection apparatus whichmonitors the number of individuals passing through and direction oftravel of each.

The data processing system may be arranged to compare a multiple-bodydeflection pattern occurring at an image line with stored deflectionpatterns, each stored deflection pattern being characteristic of asingle causative body, and thereby to be capable of resolving theunknown deflection pattern into a number of overlapping single-bodydeflection patterns. This provides the advantage of increasedflexibility by rendering the system adaptable to use in situations ofhigh traffic flow.

The data processing system includes updatable storage means arranged tomonitor the population within a designated area in accordance with thenumber of bodies passing through the detection system of the inventionand the direction of travel deduced by association of deflections ofdifferent image lines, and wherein the number of single-body deflectionpatterns resolved from the multiple-body deflection patterns isequivalent to the number of bodies passing through a light sheet andtherefore entering or leaving the designated area. This combines theadvantages of constant monitoring of the population of a restricted areawith the flexibility of adaptation for a high volume of traffic flow.

In a further embodiment, the detection system may be arranged to monitorthe presence or absence of irregularities on a surface. In thisembodiment, in the absence of an irregularity, the surface forms theobstructing surface; and in the presence of an irregularity, the surfaceof the irregularity forms the obstructing surface. The imagedintersection thus has different positions according to whether it arisesfrom the surface or the irregularity and these positions are relativelydisplaced in accordance with the height or depth of the irregularity onthe surface. This embodiment provides a further application of theinvention: examination of a surface for damaging irregularities.Specifically, the surface may be a road surface. The light source andimaging means may be located on the underside of a road vehicle, thelight source being arranged to project light in at least one light sheetonto the road surface and the imaging means being arranged to revealirregularities in the road surface as deflections in an image. Thusroads can be checked with a view to repair before excessive damage iscaused.

The light source preferably comprises solid state photoemitters arrangedto emit infrared radiation of wavelength less than 1 μm. This providesadvantages in security, convenience and accuracy. The projected lightwill not be visible to the naked eye and so avoids the distraction ofpermanent lighting. In security systems, this also reduces thelikelihood of evasive measures being taken to avoid detection.Furthermore, the system relies on good signal to noise from theprojected light striking the body beneath. This requires strong contrastand minimising the effects of ambient lighting is therefore important.Typically, the ambient lighting of buildings does not contain muchinfrared intensity in the sub-1 μm wavelength range.

A detection system arranged in a body counting implementation may becombined with a recognition system and incorporated into a seconddetection system. The recognition system is arranged to respond to thepresence of predetermined individuals, preferably by detection of aradio-frequency marker tag, and thereby to enable the second detectionsystem either to associate each count recorded by the body-countingdetection system with a member of the set of predetermined individualsor to register an intrusion. This system provides security without beingobtrusive. It can be operated with a default unlocked door or gatemaking it very attractive for fast throughput of personnel.

In another aspect, the invention provides a method of detecting objectstravelling on a surface comprising the steps of:

(a) projecting at least one sheet of light onto the surface,

(b) selectively imaging the intersection of each sheet of light with thesurface through imaging means whose optical axis is substantiallyparallel to the projection direction and offset from the projectedlight, and

(c) detecting deflections of imaged intersections of each projectedlight sheet.

This invention exploits the effects of perspective when viewing from aparticular direction. An object interrupting a light sheet can bereadily detected as an outward movement of an imaged intersection in thefield of view. It thus provides for non-intrusive detection of movementthrough the system.

The method may also comprise the steps of:

(a) associating each deflection of an imaged intersection with thepassage of an object through the projected light sheet responsible forsaid imaged intersection,

(b) responding to each deflection in accordance with the deflectionbeing caused by the passage of an object through the associated lightsheet.

This provides for an appropriate system response to be made to adeflection of an imaged intersection. This response may vary fromrecording a count to initiating a complex signal processing routine. Ittherefore confers a utility advantage in increasing the adaptability ofthe method of the invention to different purposes.

The method may further comprise the steps of:

(a) associating a deflection of one imaged intersection with asubsequent deflection of the other imaged intersection, and

(b) counting the number of deflection pairs.

This enables the direction of travel to be determined from the order inwhich the light sheets are disturbed and a count of interruptions to bekept. This provides the system with the capability for constantlymonitoring the population within a designated area.

In another aspect of the invention, a method of detecting surfaceirregularities comprises the steps of:

(a) projecting at least one sheet of light onto a surface,

(b) selectively imaging the intersection of each sheet of light with thesurface with imaging means whose optical axis is substantially parallelto the projection direction and offset from the projected light sheet,

(c) moving each light sheet and imaging means relative to the surface,and

(d) detecting deflections of imaged intersections between each lightsheet and the surface.

This aspect exploits the effects of viewing objects in perspective inorder to monitor variations in surface height. It is capable not only ofmerely detecting irregularities but also of determining their structure.Surface size is not a critical issue, the system is able to detect avariation in height down to a limiting relative change in projectiondistance. Microscopic surface irregularities can be detected althoughprojected light with correspondingly small dimensions will experiencegreater diffraction and related microscopic effects. Furthermore thereis a fundamental limitation to the smallest spot size achievable.Allowance will have to be made for this in a microscopic implementation.

In order that the invention might be more fully understood, embodimentsthereof will now be described with reference to the accompanyingdrawings in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of a detection system of theinvention.

FIG. 2 is a vertical section of FIG. 1 through an object to be detectedand a camera in the system of FIG. 1.

FIG. 3 is a representation of an image produced by the camera of FIG. 2in the absence of an object to be detected.

FIG. 4 is a representation of an image produced by the camera of FIG. 2in the presence of an object to be detected.

FIG. 5 is a representation of a data processing system for use in theinvention.

FIG. 6 is a representation of an alternative data processing system foruse in the invention.

FIG. 7 is a schematic illustration of the detection system of theinvention used in conjunction with a second detection system.

FIG. 8 is a schematic illustration of an application of the invention toroad surface mapping.

DETAILED DISCUSSION OF EMBODIMENTS

With reference to FIG. 1 a detection system of the invention arranged ina body counting implementation is indicated generally by 10. Thedetection system is located in a corridor 12 which forms an entrance toand/or exit from an area of a building with restricted accessrequirements. The corridor 12 is of height h_(c) and has ceilingindicated by lines 14 and a floor indicated by lines 16. The detectionsystem 10 generates first and second parallel sheets of light 18, 20separated by a distance 2d₁ and symmetrically disposed about adownward-looking camera 22. Bach light sheet 18, 20 is generated bysources (not shown) located on the ceiling 14 which project light ontothe floor 16. The light sheets 18, 20 are arranged to cross-section thecorridor 12. The camera 22 is located in the ceiling 14 and has anoptical axis 24 running perpendicular to the ceiling 14 and floor 16.This arrangement possesses a symmetry plane which intersects with theceiling 14 along a symmetry line 25. The system 10 is arranged to detectwhether or not a body 26 of height h_(body) is or is not positionedwithin the area of either light sheet 18, 20. In the absence of the body26, light projected along a thin slice δL1 of the first light sheet 18is reflected by the floor 16 and follows ray path P₀ to form an image inthe camera 22. If the first light sheet 18 is interrupted by the body 26then light projected along the thin slice δL1 is reflected from the headof the body 26 and follows ray path P_(body) to form an image in thecamera 22. The distance from the source of the thin slice δL1 to thecamera 22, both located in the ceiling 14, is denoted by d.

It is to be noted that neither FIG. 1 nor FIG. 2 are drawn to scale.Horizontal dimensions have been exaggerated somewhat for clarity.Typically d₁ is of the order 0.1 m and h_(c) is of the order 2.5 m.

FIG. 2 is vertical section of FIG. 1. The section is taken through thecamera 22 and the projected light slice δL1 intersecting the body 26. Inthis Figure objects and distances previously described in relation toFIG. 1 are like referenced. The line δL1 intersects the floor 16 at apoint A and the ceiling 14 at a point B. The camera optical axis 24intersects the ceiling 14 at a point C and the floor 16 at a point D.The distance between points B and C is denoted by d. The tallest pointof the body 26 interrupts the slice δL1 of light sheet 18 at a point H.A point E is located at the intersection of the optical axis 24 and aplane parallel to the ceiling 14 passing through point H. The ray lineP₀ intersects the line HE at point X. The camera 22 forms an image at aplane displaced a distance f from the plane of the ceiling 14. Thisimage plane intersects the camera optical axis 24 at a point M and theslice δL1 at a point N. The intersection of the camera image plane andthe planar section shown in this Figure is illustrated by the line NM.Along this line are shown the intersections of the image plane with raypath P_(body) at a point K and with ray path P₀ at a point J. The pointJ is located a distance d₁ from the camera axis point M and a distanced_(shift) from point K.

FIG. 3 is a representation of the image formed at the camera 22 by thelight sheets 18, 20 viewed in the perspective configuration of FIG. 1and in the absence of any interrupting body The symmetry line 25 of thesystem projected onto the image plane of the camera forms a symmetryline 40 which bisects the image plane. The image comprises two brightlines 50, 52 parallel to the symmetry line 40 which define three darkrectangular areas 44, 46, 48. The first and second bright lines 50, 52are located distances d_(i1) to the left and right respectively of thesymmetry line 40 and are the respective images of the first and secondlight sheets 18, 20. The camera axis 24 passes through a central point54 of the image. Displacement lines 56 a, 56 b, 56 c, 56 d extendradially from the central point 54 and intersect the first bright line50 at intercept points 58 a, 58 b, 58 c, 58 d respectively.

FIG. 4 illustrates schematically the image formed at the camera 22 bythe light sheets 18, 20 viewed in the perspective configuration of FIG.1 when a body 26 interrupts the first light sheet 18. The symmetry line40, central point 54 and displacement lines 56 a, 56 b, 56 c, 56 d ofFIG. 3 are represented in this figure and similarly referenced. A firstside of the image 60 corresponds to the second light sheet 20 and isunchanged from that of FIG. 3. A second side of the image 62 correspondsto the interrupted first light sheet 18 and this comprises a distortion64 of a first bright line 66 in the vicinity of the interrupting body26. Displacement lines 56 a, 56 b, 56 c, 56 d extending radially fromthe central point 54 intersect the bright line 66 at intercept points 68a, 68 b, 68 c, 68 d respectively.

With reference to these figures, the operation of the invention will nowbe described. The principle of the system is to observe an apparentdeflection of a projected beam when an object (body) appears beneath it.This occurs because of the effect of perspective on objects as viewedfrom above. The fundamental principles will be outlined first andpractical details addressed later.

Referring once more to FIG. 1, the two sheets of light 18, 20 areparallel to one another and are projected from the ceiling 14 to thefloor 16 of the corridor 12. The camera 22 is located in the ceilingmidway between the light sheets 18, 20. The camera field of viewencompasses the floor area between the light sheets, the intersection ofboth light sheets 18, 20 with the floor 16 and part of the floor areasbeyond each light sheet 18, 20. The light sheets 18, 20 are thus viewedby the camera 22 in perspective. The image formed by the camera 22 isshown in FIG. 3. Each projected light sheet 18, 20 is imaged as arespective bright line 50, 52, displaced a distance d_(i1) to the leftand right of the symmetry line 40 perpendicular to the camera opticalaxis 54. The area of the floor 16 between the two light sheets 18, 20thus forms a central dark area 44 of width 2d_(i1) in the image. Areasof the corridor 12 beyond the light sheets 18, 20 are seen by the camera22 through the light sheets 18, 20 and imaged as outer dark areas 46,48. The relationship between image distance (d_(i1)) and theperpendicular distance between the camera 22 and each light sheet 18, 20(d₁) is a function of the geometry and optics of the system. The brightlines 50, 52 correspond to the images of the intersections of the lightsheets 18, 20 and the floor 16 and are thus straight parallel lines inthe images of uninterrupted light sheets 18, 20.

Referring now to FIGS. 1 to 4, consider a person walking along thecorridor 12 and thereby passing through the light sheets 18, 20. Thereis thus a time at which a body 26 interrupts the projection of the firstlight sheet 18. In an uninterrupted arrangement, light contained withina slice δL1 of the first light sheet 18 intersects the floor 16 at pointA. The image of this point A is produced by light propagating along rayline P₀ to a point, say, 58 b on the bright line 50. At the instant atwhich the body 26 interrupts the first light sheet 18, light containedwithin the slice δL1 no longer intersects the floor 16 but isinterrupted at the top of the body's head H. The image formed in thiscase by the camera 22 is shown in FIG. 4. The end H of slice δL1 isimaged in the camera at a point 68 b on the bright line 66 by lightpropagating along ray line P_(body). In FIG. 4 the bright line 66corresponds to the bright line 50 of the uninterrupted image shown inFIG. 3. In the interrupted case however the bright line 66 comprises twodistinct sections. In a first section the bright line 66 is the image ofthe intersection of the first light sheet 18 with the floor 16. In asecond section 64, the bright line 66 is the image of the light sheetintersection with the body 26 in the region for which the light sheet 18is prevented from intersecting with the floor 16 by that body 26. Theimage of the intersection of light within the slice δL1 with an opaqueobstruction thus moves when the opaque obstruction is switched from afloor 16 to a body 26. The pattern of this motion can be deduced fromthe geometry of the set up.

In FIGS. 1 and 2, the distance from the camera 22 to a point B on theceiling 14 directly above the head H of the body 26 is d. Light rays P₀and P_(body) the slice δL1 of the light sheet 18 which is interrupted bythe body 26 and the optical axis 24 of the camera 22 are all coplanar.This plane is defined in the figures by rectangle ABCD. Points E, H, J,K, M and N are also contained within this plane. The object plane isdefined by the level of the top of the head H of the body 26 above thefloor 16. The projected light line δL1 is reflected at H in the presenceof the body 26, follows ray path P_(body) and strikes the camera imageplane at point K. If the body is absent, the light line δL1 is reflectedat A, follows ray path P₀ and strikes the image plane at point J. Thusif a body 26 interrupts projected light line δL1 the image formed at thecamera is observed to shift from J to K, a distance d_(shift). Thisdisplacement d_(shift) can be found from consideration of similartriangles CMK and CEH: $\begin{matrix}{{\frac{d_{shift} + d_{i}}{d} = \frac{f}{h_{c} - h_{body}}}{{and}\quad {substituting}\quad {for}\quad d_{i}\text{:}}{\frac{d_{i}}{d} = \frac{f}{h_{c}}}{{to}\quad {yield}\text{:}}{d_{shift} = {{fd}\quad \frac{h_{body}}{h_{c}( {h_{c} - h_{body}} )}}}} & (1)\end{matrix}$

This displacement will be in the direction from J to K i.e. radiallyoutwards from the camera axis 24.

Referring again to FIGS. 3 and 4, possible displacements of variousimage points are indicated by dashed lines 56 a, 56 b, 56 c, 56 d. Thelines 56 a, 56 b, 56 c, 56 d emanate radially from the centre 54 of theimage intersecting the bright line 50 at a series of intercept points 58a, 58 b, 58 c, 58 d. Consider, for example, the point at which lightline δL1 intersects the floor 16. This appears in the image at point 58b. If a body 26 then interrupts light line δL1 the image point 58 b willbe displaced along direction 56 b by an amount d_(shift) determined bythe height of the body 26.

The body 26 will generally create an extended blockage and neighbouringlight lines and image points will also be displaced. An example of suchdisplacements is shown in FIG. 4. Intercept points 58 a, 58 b, 58 c, 58d are displaced to points 68 a, 68 b, 68 c, 68 d respectively. Themagnitude of the displacements can be related to the height of the body26 at each interrupt point and the direction is fixed by the geometry ofthe optical system. In this way a head and shoulders profile can bededuced from the displacements.

To determine the magnitude of the displacements the origin points on thebright line 50 have to be known. This can be excessively complicated byobstruction of the line of sight between the camera 22 and body 26 byother bodies as they pass along the corridor 12. Each displacementdirection 56 a, 56 b, 56 c, 56 d has two components: one parallel to andone perpendicular to the bright line 50. The perpendicular component isstraightforward to measure; the undisplaced line 50 can be extrapolatedfrom its uninterrupted sections. In this construction, the relevantportion of the light sheet 18 appears to be displaced outwards from thesymmetry line 40 by a perpendicular distance given by $\begin{matrix}{d_{{shift}\bot}{fd}_{\bot\quad}\frac{h_{body}}{h_{c}( {h_{c} - h_{body}} )}} & (2)\end{matrix}$

where d₁ is the perpendicular distance from the camera 22 to the lightsheet 18.

Thus, a body 26 passing through the first light sheet 18 will cause areal time displacement 64 of the bright line 66 as observed by adownward-looking camera 22 whose field of view encompasses the lightsheet 18 and floor 16 intersection. The displacement pattern will bedependent on the height variation over the head and shoulders of thebody 26.

The camera 22 detects deflections of both light sheets 18, 20. Considerthe expected case of a person walking along the corridor 12 and therebyinterrupting first one and then the other light sheet 18, 20. Deflectionpatterns 64 extending outwards from the image centre are thus observedwhen the body 26 interrupts sequentially each individual light sheet 18,20. From the order of light sheet displacements the direction of travelof the body 26 can be determined. The use of multiple light sheetsprovides the direction determining capability of the invention. Inapplications for which there is no need to determine the direction oftraffic flow, for example, an intruder alarm and road surface monitoringsystem described later, a single light sheet can be used.

In an alternative embodiment of the invention the single camera 22 isreplaced by a number of cameras arrayed along the symmetry line 25 ofthe ceiling. A single camera places extreme requirements on the width ofits field of view. To function effectively the field of view mustencompass the entire width of the light sheet at the level of a typicalbody height i.e. of order 3 m at a distance of 0.6 m. This basicallyrequires a single camera to possess a very wide field of view. Multiplecameras allow the invention to cover the entire width of the lightsheets with reduced field-of-view cameras. The problem of obscuration byone body of a second more distant body is also likely to arise morefrequently when only one line of sight is available to a single camera.The multiple views afforded by a number of cameras can significantlyreduce counting errors arising from this source.

The two light sheets 18, 20 may be formed in a number of ways. Ideallystrip sources are located in the ceiling 14 to provide for downwardslight propagation across the entire width of the corridor 12. As analternative, each sheet 18, 20 is formed by at least one laterallyfanned beam which is collimated in the longitudinal direction. Fanningin the lateral direction is achieved by a cylindrical lens. A furtherembodiment is provided by multiple point sources fanned to a lesserextent in the lateral direction and collimated in the longitudinaldirection. The longitudinal collimation of these embodiments is notcritical. Good results are achieved if the light sheets 18, 20 arerelatively narrow at the typical height of the bodies'of the order 5 mmor smaller—although standard image signal processing techniques can beused to find the centre line of a wider imaged beam.

In a further embodiment, fanning is neglected entirely and aone-dimensional array of collimated beams employed. The bright lines 50,66 of the images shown in FIGS. 3 and 4 will then be replaced by aseries of bright dots. The spacing between each beam of the array shouldbe small enough to prevent intruders by-passing the light detectionsystem. The image signal processing problem can be simplified with thisembodiment because the locus of deflection of each dot is known (i.e.radially outward from the zero deflection point with origin at the imagecentral point 54). Thus only a limited number of pixels need to besearched in locating each dot. For this reason an array embodiment maybe preferred over a sheet beam implementation. This embodiment may alsobe favoured if it is necessary or desirable to use a less intense lightsource which precludes fanning.

FIGS. 5 and 6 illustrate the camera 22 connected to a data processingsystem 80. The data processing system 80 is arranged to monitor andinterpret the image observed by the camera 22.

FIG. 5 illustrates a first embodiment of the data processing system 80.The camera reading is input to an image processor 82 which records,processes and digitises the deflection pattern 64. A counting unit 84 isarranged to count the number of such deflection patterns occurring ineach light sheet 18, 20. An interpreter 86 links disturbances from thesame body 26 passing through both light sheets 18, 20 and thus derivesdirection of travel. A memory 88 provides the interpreter 86 withinformation about the numbers of people currently inside the restrictedarea.

FIG. 6 illustrates a second embodiment of the data processing system 90.Certain components of FIG. 6 perform the same functions as components ofFIG. 5 and these are referenced similarly. The image output from thecamera 22 is passed to an image processor 82 and counting unit 84. Asecond interpreter 92 receives a processed digitised image of thedisturbance 64 and a signal from the counting unit 84. The interpreter92 is further arranged to have access to a second memory 94 whichcontains digital images of head and shoulder deflection profiles 64.This second memory 94 is also arranged to keep an individual populationrecord of numbers within the restricted site.

Referring once more to FIG. 5, the apparatus of this embodiment isarranged to keep a constant check on the number of persons within therestricted area guarded by the detection system of the invention 10. Theinterpreter 86 receives information from memory 88 detailing the currentoccupancy of the restricted area. A body entering the detection system10 from a particular direction will initiate a count within the countingunit 84 indicating that a first light sheet 18, 20 has been disturbed.The interpreter 86 will look for the complementary count from the otherlight sheet 20, 18 as the body exits the detection system 10. Thus theinterpreter 86 has knowledge of a body passing the detection system 10and of the direction in which it travelled. The interpreter 86 thenadjusts the number of persons in the restricted area accordingly, andupdates the value stored in memory 88. The system is now prepared tocount a second body passing through the detection system 10.

The image processor 82 is arranged to perform standard image processingfunctions. In any embodiment of the invention with multiple camerasaligned along the symmetry line 25 of the ceiling 14, the imageprocessing includes a merger stage during which a single image of thedeflection is produced from image overlaps when one body falls withinthe field of view of more than one camera.

The way in which the interpreter 86 associates two disturbances indifferent light sheets with the same travelling body 26 can be any oneof a number of variants depending on the accuracy required. By way ofexample only, and not limiting the scope of the invention, some methodsare illustrated below.

First, there need not be any particular association made at all. If acount is recorded at one light sheet 18, 20 then the first subsequentdisturbance of the other light sheet 20, 18 is ignored and the body isassumed to be travelling in a direction from the first to the secondlight sheet. This arrangement will misassign situations in which thesecond light sheet is disturbed by a second person entering thedetection system 10 before the first person exits. However the totalnumber of people within the restricted area will only be wrong for thetime between the measured exit and the true exit. This arrangement maybe acceptably accurate if it can be certain that the camera 22 detectsall disturbances to the light sheets 18, 20. It will have most use ifapproximate numbers are required from a high traffic flow at a roughlyconstant speed. The camera 22 will have to be adjusted such that theframe rate is sufficiently high for a fast passage of the beam to bedetected as a disturbance and not averaged out in a single frame. Thelight sheet separation (2d₁) can be adjusted to approximately thedistance travelled by the body 26 in one image frame of the camera. Thebody 26 will then be visible beneath the two light sheets 18, 20 duringconsecutive frames. This will allow determination of the direction oftravel and if necessary approximate velocity. Assuming a maximum bodyvelocity of 6 ms⁻¹, then a sheet separation of 0.25 m is the minimumwhich should be used with a typical camera frame rate of 25 Hz.

One method of association is based on an estimate of the average humanwalking speed. From this figure and an original disturbance at one lightsheet 18, 20, the frame in which the associated disturbance of the otherlight sheet 20, 18 is expected to be observed is estimated: Theinterpreter 86 is arranged to recognise the actual disturbance of thesecond light sheet 20, 18 which is temporally closest to the expecteddisturbance. This actual detected disturbance is then assigned to be theassociate of the original disturbance to the first light sheet 18, 20.Time limits can be set and if no actual disturbance to the second lightsheet 20, 18 is detected during this limit then the camera 22 is assumedto have missed the exit from the system 10. This will give rise toinaccuracies as the camera 22 is equally likely to miss the entry to thedetection system 10 as the exit and serious miscounts could result.However if heavy traffic is anticipated, then this may provide aninexpensive implementation of the invention.

A more accurate method of association is based on a process of patternmatching. In this embodiment, the deflection pattern 64 of a first lightsheet 18, 20 is digitised by the image processor 82 and passed to thememory 88. In embodiments of the invention for which data reduction isnecessary, the image processor 82 is also arranged to account for adisturbance being within the field of view of more than one camera. Theinterpreter 86 is then arranged to match the deflection patterns of thesecond sheet 20, 18 with those of the first sheet stored in the memory88 and associate accordingly. In this way information about both thedirection and the speed of travel can be extracted by the interpreter86. The number of parameters used to register a match can be adjustedaccording to the accuracy and speed requirements of the application.Deflection patterns 64 can be matched statistically across the wholedisturbance profile or simply to a single parameter such as maximumheight of the interrupting body 26. In this embodiment it is necessaryto have a sufficiently high camera frame rate in order to detect anacceptable majority of the light sheet disturbances.

Referring once more to FIG. 6, there is shown a second embodiment of thedata processing system 90. In this arrangement the detection system 10is designed to perform a more extensive process of pattern matching.This permits the resolution of individual head and shoulder profilesfrom overlapping disturbances and hence enables movements of largenumbers of people into and out of the restricted area to be monitored.The image output from the camera 22 is passed to an image processor 82and counting unit 84. An interpreter 92 receives a digitised image ofthe disturbance 64 and a signal from the counting unit 84. Theinterpreter 92 is further arranged to have access to a memory 94 whichcontains digital images of sample deflection profiles 64. The memory 94is also arranged to keep a record of population within the restrictedarea.

In this embodiment multiple bodies pass together through the lightsheets 18, 20. A single body 26 passing through a light sheet 18 causesa real-time disturbance of the associated bright line 66 of the image.The deflection pattern 64 is characteristic of a head and shouldersprofile. If two well separated bodies pass simultaneously through thelight sheet 18 then the bright line will exhibit two deflection patternswith readily discernible onset and completion which are easily resolved.However if one body is partly or completely behind the other then thetwo deflection patterns are not so readily separable. In this embodimenttherefore, the image processor 82 records, processes and digitises acomposite deflection profile 64 resulting from multiple bodies passingthrough a first light sheet. The interpreter 92 receives the digitisedimage and attempts to match it with a combination of individualdeflection profiles stored within the memory 94. It also performs astatistical test on close-matching solutions to determine the best fit.The counting unit 84 then registers a count in accordance with thenumber of individuals deduced by the interpreter 92. This embodiment ofthe invention may be suitable for assessing attendance at sportingevents at which the major traffic component flows in one direction only.Alternatively the direction of travel is determined by one of thetechniques detailed for the processing system of FIG. 5.

The possibility has also been considered of using the interpreter 92 toidentify certain individuals from their characteristic deflectionprofiles. This would involve installing a complete database ofdeflection profiles of all persons permitted access to the restrictedsite and setting the interpreter 92 to perform a pattern matchingexercise. The level of pattern matching adopted is variable from astraightforward height classification to carrying out an extensiveprofile fit. If the match between an observed and expected deflection isoutside a certain limit, no identification is made and a suitableintruder alarm is activated. If a good fit is registered then theobserved deflection pattern is associated with the movement of theperson identified as producing that fit. Such a system would have to beable to cope with the variety of postures adopted by a single person andthe likely reduction of available image information due to obscuration.

The image processing can be performed more effectively if the camera 22is focused correctly. The most important part of a body 26 seen by thecamera 22 is the head and shoulder profile, particularly in embodimentsfor which pattern recognition is employed. It follows therefore that thecamera 22 is arranged to focus on the typical height of thebodies—generally 0.6 m to 0.9 m from the camera 22.

FIG. 7 illustrates a further embodiment 100 of the detection system ofthe invention 10. It incorporates a camera 22 arranged for light sheetobservation as in the previous examples. In combination with this, aradio-frequency tagging detection system 102 is arranged to operate atthe same location. The outputs from both detection systems are input toa digital processing system 104.

In the embodiment shown in FIG. 7, the detection system of the invention10 is arranged to detect all persons passing through a corridor 12. Atagging detection system 102 is also arranged to acknowledge a personpassing through the same corridor 12 only if they are wearing aparticular tag identifier. Examples of radio-frequency tagging detectionsystems are known in the prior art and will not be described here. Thedigital processing system 104 will thus receive a signal from thedetection system of the invention 10 if any body passes through. If theprocessing system 104 also receives a signal from the tagging detectionsystem 102 then that body is allowed to pass freely. If, however, nosignal is received from the tagging detection system 102 then the entryof an intruder is detected and a suitable alert issued.

FIG. 8 illustrates a second implementation of the detection system ofthe invention 110. Light sheets 118, 120 are generated from sourcesattached to the underside of a road vehicle 116. As before, a camera 122is located midway between these light sheets 118, 120 and also attachedto the underside of the vehicle 116. The vehicle 116 is driven along aroad surface marked with potholes and similar road irregularities 124.

The vehicle 116 with the attached detection system is driven along theroad. Whenever a light sheet 118, 120 intersects with an irregularity124 in the road surface a deflection of a bright line in the imageoccurs as described earlier. The deflection profile is indicative of theshape and depth or height of the irregularity 124. Data processingequipment can be stored within the road vehicle 116 in order to providea readily assessable interpretation of the image variations. A secondlight sheet 120 in this implementation is optional given that thedirection of vehicle motion is known. The data gathered can however beused to gain a more accurate picture of the interrupting irregularity124. The invention can thus be used to rapidly map and assess roadsurfaces for repair.

A further application of the detection system of the invention is inrecognising the orientation of objects on a conveyor belt or productionline. The light sheets are projected onto the conveyor belt and theintersections imaged by an appropriately placed camera. The transverseheight profile can be determined from each single-frame deflectionpattern and the longitudinal profile from the change in deflectionpattern across sequential frames. Objects correctly oriented will giverise to a characteristic series of deflection patterns as they passthrough the intersections on the conveyor belt. Misaligned objects canthen be detected by their unexpected deflection patterns and reoriented.

Theoretically there are no reasons per se for restricting the lightforming the light sheets to a particular wavelength. However, theapplication to which the invention is put may favour a particularspectral range over others. The invention uses an active optical signaland relies on a good signal to noise ratio from the projected light beamstriking the object beneath. This requires strong contrast andminimising the effects of ambient lighting is important. Furthermore inthe body counting implementation it is advantageous to have inobtrusivelighting which effectively excludes the visible spectrum fromconsideration. Strong contrast in a corridor of a building is achievedby using narrow spectral response short-wave infrared (sub 1 μm)radiation generated by solid state photoemitters. This wavelength rangeis detected by silicon CCD cameras, with enhanced red response ifnecessary. The camera is narrow-band filtered to the wavelength of theemitter in order to reduce the energy detected from background lighting.

What is claimed is:
 1. A detection system for detecting movement of anobject relative to the system, said system comprising: a light sourcefor projecting light in a projection direction to form an intersectionwith an obstructing surface, the intersection extending across adetection zone of the system and comprising a line of light which is oneof a continuous line of light and a line defined by a series of separateregions of light; imaging apparatus comprising at least one camera forproviding an image of the intersection of said light with theobstructing surface, the imaging apparatus: responsive to said light,having an optical axis which is offset from the projection direction,producing a line image of a profile of the intersection of the projectedlight with the obstructing surface in an image plane of the camera, andwhere at least part of the obstructing surface is not that of areference surface, at least part of its imaged profile is displaced inthe image plane relative to an image position corresponding tointersection of projected light with the reference surface; imageprocessing apparatus, said image processing apparatus: responsive todeflection of the imaged profile of the light intersection caused byrelative movement of an object through the detection zone providing achange of obstructing surface position, and detecting a line imagedeflection pattern produced by the obstructing surface profile relativeto image deflection produced by at least one elongate reference profile;and the deflection pattern comprising in perpendicular displacements(d_(shift 1)) of line image components in a line image of theobstructing surface profile from positions of corresponding line imagecomponents in a reference line image, said displacements being describedat the imaging apparatus' image plane by an equation as follows:$d_{{shift}\bot} = {{fd}_{\bot}\frac{h_{body}}{h_{c}( {h_{c} - h_{body}} )}}$

where d₁ is the perpendicular distance from the imaging apparatus to alight sheet associated with the line image of the obstructing surfaceprofile,f is the focal length of the imaging apparatus, h_(C) is thedistance between the imaging apparatus and the bounding surface andh_(body) is a parameter indicating the body surface's distance from thereference surface at each point that it intersects the light sheet.
 2. Adetection system for detecting movement of objects relative to thesystem, said system comprising: a light source for projecting at leastone substantially planar sheet of light in a projection direction toform an intersection with an obstructing surface, the intersectionextending across a detection zone of the system and comprising at leastone line of light which is one of a continuous line of light and a linedefined by a series of separate regions of light; imaging apparatuscomprising at least one camera for providing an image of theintersection of said light with the obstructing surface, the imagingapparatus: responsive to said light, having an optical axis which isoffset from the projection direction, producing a line image of aprofile of the intersection of the projected light with the obstructingsurface in an image plane of the camera, and where at least part of theobstructing surface is not that of a reference surface, at least part ofits imaged profile is displaced in the image plane relative to an imageposition corresponding to intersection of projected light with thereference surface; where for each projected light sheet, in the absenceof an interrupting object, the reference surface providing anobstructing surface and the line image is located in a first position inthe image; and for each projected light sheet, in the presence of anumber of objects interrupting the light sheet, the combined surfaces ofthe objects and any intermediate regions of the reference surfaceproviding an obstructing surface and a line image associated with theinterrupted light sheet exhibiting a multiple-object deflection patterncharacteristic of profiles of objects passing through the light sheet:and image processing apparatus: responsive to deflection of the imagedprofile of the light intersection caused by relative movement of anobject through the detection zone providing a change of obstructingsurface position, detecting a line image deflection pattern produced bythe obstructing surface profile relative to image deflection produced byat least one elongate reference profile, and comparing themultiple-object deflection pattern with stored deflection patterns, eachstored deflection pattern being characteristic of a single object, andthereby is capable of resolving the multiple-object deflection patterninto a number of overlapping single-object deflection patterns.
 3. Adetection system for detecting movement of objects relative to thesystem, said system comprising: a light source for projecting twosubstantially planar sheets of light in a projection direction to formintersections with an obstructing surface, the intersections extendingacross a detection zone of the system and comprising two lines of light,each line of light comprised of continuous line of light and a linedefined by a series of separate regions of light; imaging apparatuscomprising at least one camera for providing an image of theintersection of projected light from the light source with theobstructing surface, the imaging apparatus: responsive to projectedlight, having an optical axis which is offset from the projectiondirection, producing a line image of a profile of the intersection ofthe projected light with the obstructing surface in an image plane ofthe camera, and where at least part of the obstructing surface is notthat of a reference surface, at least part of its imaged profile isdisplaced in the image plane relative to an image position correspondingto intersection of projected light with the reference surface; for eachprojected light sheet interrupted by a number of objects, the combinedsurfaces of the objects and any intermediate regions of the referencesurface forming the obstructing surface and the line image associatedwith the interrupted light sheet exhibiting a multiple-object deflectionpattern characteristic of profiles of objects passing through the lightsheet; and image processing apparatus: responsive to deflection ofprojected light intersections caused by relative movement of objectsthrough the detection zone providing a change of obstructing surfaceposition, and detecting a line image deflection pattern produced by theobstructing surface profile relative to image deflection produced by atleast one elongate reference profile; comparing the multiple-objectdeflection pattern with stored deflection patterns, each storeddeflection pattern being characteristic of a single object, and theimage processing apparatus thereby being capable of resolving themultiple-object deflection pattern into a number of overlappingsingle-object deflection patterns; and monitoring population within adesignated area in accordance with number of objects passing through thesystem, to deduce direction of travel by association of deflections ofdifferent image lines, and resolve single-object deflection patternsfrom multiple-object deflection patterns for deriving the number ofobjects passing through a light sheet and therefore entering or leavingthe designated area.